The present invention relates to a biaxially stretched polypropylene film, and more especially to a composite biaxially stretched polypropylene film comprising two layers and having one surface which is rougher than the other surface. In a metallized form, the film is used for the manufacture of capacitors.
Biaxially stretched polypropylene films are employed as sealable films for packaging purposes; see, e.g., German Auslegeschrift No. 1,694,547, and they are also used for electrical applications, e.g., as metallized films for the manufacture of capacitors.
A generally known problem encountered in the processing of smooth polypropylene films is their blocking propensity, for example, in the course of metallizing, cutting or winding up the films. In particular, these films cannot be used in high-speed capacitor winding machines. To solve this problem, polypropylene films with rough surfaces are produced, and these can be processed without difficulty.
German Offenlegungsschrift No. 2,740,237 discloses a process for the manufacture of a polypropylene film with rough surfaces, in which the desired roughness is obtained by choosing predetermined crystallizing temperatures and cooling periods.
German Offenlegungsschrift No. 2,553,693 describes a process for the manufacture of rough polypropylene films, in which a fibrillar network structure is built up on the film surface by applying specific stretching temperatures and stretching conditions. This network structure has the purpose of providing a better absorption of the insulating oils used in the production of capacitors.
A similar process for preparing rough electrical insulating films has been disclosed in German Offenlegungsschrift No. 2,722,087. Also in this case, special stretching conditions and stretching temperatures are employed to produce .beta.-shaped crystals in the film surface. These crystals have an average diameter exceeding 7 .mu.m, and the density of distribution of the crystals must amount to more than 5,000 crystals per cm.sup.2 (in the case of spherulites) and to more than 1,000 crystals per cm.sup.2 (in the case of rod-shaped crystals). The rough surface likewise serves to enhance the absorptivity toward insulating oils, thus preventing failures or breakdowns due to corona discharges in the capacitor.
German Offenlegungsschrift No. 2,601,810 is also concerned with an improvement of the impregnation of capacitor rolls. For that purpose, particular temperature conditions are chosen in the manufacture of tubular films, as a result of which the films are roughened, i.e., spherulites of the type III crystalline structure are generated.
Due to the steep characteristic curve of the kinetics of crystallization, however, the processes mentioned have the common disadvantage that they must adhere to very special process conditions, so that they are technically rather involved and extremely susceptible to failures caused by external influences, e.g., by air convection. In addition, in the films manufactured according to these processes, attention is principally paid to the absorptivity toward insulating oils and less to the electrical properties.
As is known, the electrical properties of biaxially stretched polypropylene films depend strongly on the structure of their surface. On the one hand, ideal electrical properties are observed in polypropylene films having very smooth and planar parallel surfaces, but these films give rise to the above-described problem of blocking during processing. On the other hand, the good electrical properties of polypropylene films decrease with an increasing roughness of their surfaces. It is above all the so-called "life test", i.e., the loss of capacitance of a capacitor as a function of time [.DELTA.C/C=f(t)], which reveals a clear dependence on surface roughness.
Furthermore, non-uniform coating thicknesses are produced when vacuum metallizing polypropylene films having rough surfaces, because the metal coating is thinner at the sides of the surface peaks. As a result, a so-called "metal separation" occurs to an increasing degree in the capacitor, which causes a reduction of capacitance or a failure of the capacitor.